1. Field of the Invention
The present invention relates generally to a display device using a light modulator and, more particularly, to a display device using a light modulator and having an improved numerical aperture of an after-edge lens system, in which the numerical aperture of the lens system, which is used to focus diffracted light beams having + and − orders that are formed by the light modulator, is significantly reduced.
2. Description of the Related Art
With the development of micro technology, so-called Micro-Electro-Mechanical System (MEMS) devices and small-sized apparatuses into which MEMS devices are assembled are attracting attention.
An MEMS device constitutes a microstructure on a substrate, such as a silicon substrate or glass substrate, and is a device that is formed by electrically and mechanically connecting a driving body for outputting mechanical driving force to a semiconductor integrated circuit for controlling the driving body. A basic feature of the MEMS device is that the driving body having a mechanical structure is placed in a portion of the MEMS device. The driving body is electrically operated using Coulomb's force generated between electrodes.
FIGS. 1 and 2 show a representative construction of an optical MEMS device that uses the reflection or diffraction of light and is applied to an optical switch and an optical modulation element.
An optical MEMS device 1 shown in FIG. 1 includes a substrate 2, a substrate-side electrode 3 formed on the substrate 2, a crossbeam 6 provided with a driving-side electrode 4 that is disposed parallel to the substrate-side electrode 3, and a support 7 configured to support one end of the crossbeam 6. The crossbeam 6 and the substrate-side electrode 3 are electrically insulated from each other by an aperture 8 therebetween.
The substrate 2 may be formed of a substrate in which an insulation film is formed on a semiconductor substrate such as a silicon (Si) or gallium arsenide (GaAs) substrate, or an insulation substrate such as a glass substrate. The substrate-side electrode 3 may be formed of a poly-crystal silicon film doped with an impurity, or a metallic film such as a Cr deposition film. The crossbeam 6 includes an insulation film 5 such as a silicon nitride film (SiN film), and a driving-side electrode 4 composed of, for example, an aluminum (AL) film that is formed on the insulation film 5 to have a film thickness of about 100 nm, and that is adapted to also function as a reflective film. The crossbeam 6 is mounted in a cantilever manner such that only one end thereof is supported by the support 7.
In the optical MEMS device 1, the crossbeam 6 is displaced by electrostatic attraction or electrostatic repulsion that is generated between the crossbeam 6 and the substrate-side electrode 3 by voltage applied to the substrate-side electrode 3 and the driving-side electrode 4. For example, the crossbeam 6 is displaced between an equilibrium state and a tilt state with respect to the substrate-side electrode 3, as shown in the solid and dotted lines of FIG. 1.
Another optical MEMS device 11 shown in FIG. 2 includes a substrate 12, a substrate-side electrode 13 formed on the substrate 12, and a beam 14 formed across the substrate-side electrode 13 in the form of a bridge. The crossbeam 14 and the substrate-side electrode 13 are electrically insulated from each other by an aperture 10 that is positioned therebetween.
The crossbeam 14 includes a bridge member 15 formed on the substrate 12 across the substrate-side electrode 13 in the form of a bridge and formed of, for example, an SiN film, and a driving-side electrode 16 formed on the bridge member 15 parallel to the substrate-side electrode 13, adapted to serve as a reflective film, and formed of, for example, an Al film having a film thickness of about 100 nm. The substrate 12, the substrate-side electrode 13 and the crossbeam 14 may have the same construction and material as described in conjunction with FIG. 1. The crossbeam 14 is mounted in a so-called cantilever manner such that only one end thereof is supported by the support 7.
In this optical MEMS device 11, the crossbeam 14 is displaced by electrostatic attraction or repulsion that is generated between the MEMS device and the substrate-side electrode 13 by voltage applied to the substrate-side electrode 13 and the driving-side electrode 16. For example, the crossbeam 6 can be displaced between an equilibrium state and a concave state with respect to the substrate-side electrode 3, as shown by the solid and dotted lines of FIG. 2.
The optical MEMS devices 1 and 11 can be applied as an optical switch having a switch function, in which, when light is radiated onto the surface of each of the driving-side electrodes 4 and 16 also serving as optical reflective films, reflected light is detected in one direction based on the fact that the reflection direction of light varies depending upon the driving position of the crossbeam 6 or 14.
Furthermore, the optical MEMS devices 1 and 11 can be applied as an optical modulation element that modulates the intensity of light. In the case where the reflection of light is used, the intensity of light is modulated by vibrating the crossbeams 6 and 14 based on the amount of reflected light in one direction per unit time. The optical modulation element uses so-called time modulation.
In the case where the diffraction of light is used, an optical modulation element is formed by parallelly arranging a plurality of crossbeams 6 with respect to common substrate-side electrodes 3 and 13, and the height of driving-side electrodes also serving as optical reflective films is changed by the approach and separation of an alternate crossbeam 6 or 14 to and from the common substrate-side electrodes 3 and 13. The intensity of light, which is reflected by the driving-side electrodes, is then modulated via diffraction. This optical modulation element employs so-called spatial modulation.
FIG. 3A and FIG. 3B shows the construction of a Grating Light Valve (GLV) device that was developed by Silicon Light Machines (SLM) Corporation as an optical intensity conversion device for a laser display, i.e., a light modulator.
As shown in FIGS. 3A and 3B, in the GLV device 21, a common substrate-side electrode 23 made of a high melting point metal, such as tungsten or titanium, and a nitride film thereof or a thin polysilicon film is formed on an insulation substrate 22 such as a glass substrate. A plurality of, in this example, six beams 24 (241, 242, 243, 244, 245 and 246) are formed parallel to each other across the substrate-side electrode 23 in the form of a bridge. The substrate-side electrodes 23 and the crossbeams 24 have the same construction as described in conjunction with FIG. 2. That is, a crossbeam 24 is fabricated by forming a driving-side electrode 26, which also serves as a reflective film and is made of an Al film having a thickness of about 100 nm, on the surface of a bridge member 25 that is parallel to the substrate-side electrode 23 and is formed of a SiN film.
A bridge member 25 and crossbeams 24 composed of the driving-side electrodes 26 and adapted to also serve as a reflective film constitute a part that is commonly called a ribbon.
The Al film used as the material of the driving-side electrodes 26 of the crossbeams 24 is a desired material for optical elements because (1) it can be formed relatively easily, (2) the wavelength dispersion of reflectance in a visible light region is small, (3) a natural Al oxide film created on the surface of an Al film serves as a protection film to protect a reflective surface.
Meanwhile, a SiN (silicon nitride) film constituting the bridge member 25 is a SiN film formed by a reduced pressure CVD method. The SiN film has physical properties, such as strength and a coefficient of elasticity, which are suitable for the mechanical driving of the bridge member 25.
If a small voltage is applied between the substrate-side electrode 23 and the driving-side electrodes 26 also serving as the reflective film, the crossbeams 24 approach the substrate-side electrode 23 due to the above-described electrostatic phenomenon. If the application of the voltage is stopped, the crossbeams 24 return to their original state.
The GLV device 21 alternately changes the height of the driving-side electrode 26 also serving as the optical reflective film via the approach and separation operations of the crossbeams 24 with respect to the substrate-side electrodes 23 (i.e., the approach and separation operations of the crossbeams), and modulates the intensity of light, which is reflected from the driving-side electrodes 26 by diffraction (one optical spot is projected for all the six beams 24).
The dynamic characteristics of the crossbeams that are driven using electrostatic attraction and repulsion are mostly determined by the material properties of a SiN film formed by the CVD method. The Al film usually serves as a mirror.
FIG. 4 is a sectional view illustrating a depression-type diffractive light modulator using a piezoelectric material, which was developed by Samsung Electro-Mechanics.
Referring to FIG. 4, the depression-type thin film piezoelectric light modulator developed by Samsung Electro-Mechanics includes a silicon substrate 40 and a plurality of elements 42a to 42n. 
In this case, the elements 42a to 42n have uniform widths, are alternately arranged, and form the depression-type thin film piezoelectric light modulator. Alternatively, the elements 42a to 42n may be alternately arranged to have different widths and may form the depression-type thin film piezoelectric light modulator. Meanwhile, the elements 42a to 42n may be spaced apart from one another by regular intervals (each of the intervals is substantially identical to the width of the elements), in which case a micromirror layer formed on the entire top surface of the silicon substrate 40 diffracts incident light by reflecting the light.
The silicon substrate 40 has a depressed portion to provide an air gap to the elements 42a to 42n. An insulation layer 41 is deposited on the top surface of the silicon substrate 40. The ends of the elements 42a to 42n are attached to both ends of the silicon substrate 40 beside the depressed portion.
The elements 42a (although only the element 42a is described herein, the remaining elements 42b to 42n have the same construction and operation) has a rod shape. The element 42a includes a bottom support 43a, the bottom surfaces of both ends of which are attached to both ends of the silicon substrate 40 beside the depressed portion of the silicon substrate 40 so that the center portion of the element 42a can be spaced apart from the depressed portion of the silicon substrate 40, and the center portion of which is located above the depressed portion of the silicon substrate 40 and can move perpendicularly.
The element 42a further includes a bottom electrode layer 44a formed on the left side of the bottom support 43a and adapted to provide piezoelectric voltage, a piezoelectric material layer 45a formed on the bottom electrode layer 44a and adapted to contract and expand and, thus, generate perpendicular driving force when voltage is applied to both ends thereof, and a top electrode layer 46a formed on the piezoelectric material layer 45a and adapted to provide piezoelectric voltage to the piezoelectric material layer 45a. 
The element 42a further includes a bottom electrode layer 44a′ formed on the right side of the bottom support 43a and adapted to provide piezoelectric voltage, a piezoelectric material layer 45a′ formed on the bottom electrode layer 44a′ and adapted to contract and expand and, thus, generate perpendicular driving force when voltage is applied to both ends thereof, and a top electrode layer 46a′ formed on the piezoelectric material layer 45a′ and adapted to provide piezoelectric voltage to the piezoelectric material layer 45a′. 
Korean Pat. Appl. No. 2004-74875. filed Sep. 18. 2004, discloses a projection-type light modulator in detail, in addition to the depression-type light modulator described above.
FIG. 5 illustrates an example of an optical apparatus, which employs a GLV device, that is, an optical modulation device, using a MEMS device, or the piezoelectric diffractive light modulator made by Samsung Electro-Mechanics. In this example, a case where the optical apparatus is applied to a laser display is described.
A laser display 51 related to the example is used as a projector for a large screen, more particularly, a digital image projector, or as an image projection device for a computer.
As shown in FIG. 5, the laser display 51 includes a laser light source 52, a mirror 54 disposed opposite the laser light source 52, an illumination optical system (lens group) 56 and a GLV device or a piezoelectric diffractive light modulator 58 that serves as an optical modulation element.
The laser display 51 further includes a mirror 60 for reflecting laser light the optical intensity of which is modulated by the GLV device or piezoelectric diffractive light modulator 58, a projection lens 62, a filter 64, a diffuser 66, a mirror 68, a galvano scanner 70, a projection optical system (lens group) 72 and a screen 74.
In the conventional laser display 51, laser light radiated from the laser light source 52 is incident on the GLV device or piezoelectric diffractive light modulator 58 through the mirror 54 from the illumination optical system 56.
Further, the laser light is spatially modulated by being diffracted by the GLV device or piezoelectric diffractive light modulator 58, reflected by the mirror 60, and then separated by the projection lens 62 on a diffraction order basis. Thereafter, only signal components are extracted from the laser light by the filter 64.
Thereafter, the laser spectrum of the image signal is reduced by the diffuser 66, and spread over the space by the galvano scanner 68 synchronized with the image signal through the mirror 68, and is then projected by the projection optical system 70 onto the screen 72.
According to the prior art, if the distance between the diffraction gratings of the diffractive light modulator is shortened, the diffraction angle increase. As a result, the Numerical Aperture (NA) of the lens system located behind the projection lens increases.
FIG. 6A shows an example of a prior art optical system having a high diffraction angle. If the diffraction angle θ is large, the NA of the projection lens increases.
FIG. 6B is a view illustrating another example of a prior art optical system having a high diffraction angle. If the incidence angles of illumination beams are different but the diffraction angle θ is large, the NA of the projection lens increases, which is the same as in the embodiment of FIG. 6A. As described above, when the NA of the lens system, such as a projection lens, located behind the diffractive light modulator increases, there are many limitations in designing the laser display. Further, if the NA is large, there is great difficulty in designing a lens because F/# is low.
Moreover, light progressing toward the center of the after-edge lens system, such as the projection lens, forms a radical axis optical system, which improves the performance of the lens. However, the structures of FIGS. 6A and 6B are disadvantageous in that the central portion of the after-edge lens system is not used but the peripheral portion of the after-edge lens system is used, so that it is difficult to expect good performance.